1. Field of the Invention
The present invention relates to electrical conduits and raceways and, more particularly, to such conduits and raceways as are resistant to physical damage. More specifically, the present invention relates to electrical conduits and raceways having a metal lining and an outer layer of PVC or like plastics.
2. Description of the Related Art
Underground installations of electrical raceways, conduit or duct banks for current carrying conductors are designed to be waterproof and corrosion proof. The majority are now fabricated out of PVC (Polyvinylchloride), and used to construct direct burial raceways that can run horizontal or vertical. When a change of course is required, bends are used to go left or right for horizontal runs and bends permit a change in direction of up and down for vertical runs.
Electricians are bound by the National Electrical Code to insure the safety and quality of electrical installations, with the Code offered for use in law and for regulatory purposes in the interest of life and property protection. As provided in ARTICLE 300.5 (4) ENCLOSURE OR RACEWAY DAMAGE, where the enclosure or raceway is susceptible to being physically damaged, the conductors shall be installed in one of the following: rigid metal conduit; intermediate conduit; Schedule 80 rigid nonmetallic conduit; or an equivalent.
Another set of requirements is present where it is anticipated that pull string will be used to pull rope through the raceway for wire pulls. In such environments, the materials used to fabricate the conduit must prevent the string or rope from cutting or burning through the interior radius of the bends. Openings resulting from such internal radius failure will expose the current conductors to water and corrosive elements outside of the raceway, as well as adversely impact the pulling process.
Overhead or exposed raceways are not subjected to the harsh environment of underground installations, permitting the use of Electrical Metallic Tubing (EMT), which has thinner walls than the rigid conduit used to construct all bends used in underground applications. Of course, the EMT walls in above-ground installations also must be sufficiently thick to permit wire pulls without encountering inner radius failure. Structurally suitable for such above ground applications, EMT is more cost effective than its underground counterpart: the thicker-walled, Galvanized Steel Rigid bends.